A recently developed type of secondary cell comprises: (1) a cell container; (2) a cation-permeable barrier to mass liquid transfer in the form of a solid electrolyte tube disposed within said container such that a first reaction zone is located within said cation-permeable barrier and a second reaction zone is located between the outer surface of said cation-permeable barrier and the inner surface of said container, one of said reaction zones being an anodic reaction zone and the other said reaction zone being a cathodic reaction zone; (3) an anodic reactant within said anodic reaction zone comprising a molten alkali metal anode-reactant such as sodium in electrical contact with an external circuit; (4) a cathodic reactant disposed within said cathodic reaction zone and comprising a liquid electrolyte such as sulfur and molten polysulfides, which is electrochemically reversibly reactive with said alkali metal; and (5) a cathodic electrode comprising a porous conductive material disposed within said cathodic reaction zone, connected to said cation-permeable barrier and adapted to be connected to said external circuit.
As used herein, the term "reactant" is intended to mean both reactants and reaction products.
In the method which has been conventionally used to prepare such cells, the anodic reactant, or sodium, and the initial cathodic reactant, or sulfur, have been added to the two reaction zones of the cell while in a molten state. Normally, the molten sulfur is added to the cathodic reaction zone through a fill spout which is pinched off and sealed after filling the cathodic reaction zone. Thereafter, the assembled cell subassembly is evacuated and melted sodium is flowed into the cell through a fill spout, which also is subsequently sealed off. If the cell is not to be employed immediately, the prepared cell is next "frozen" so as to solidify the reactants. Both this method and the resultant cell suffer from some rather serious disadvantages. First, the method is undesirably dangerous because of the use of molten reactants, thus exposing those preparing the cells to undue danger. Secondly, the preparation of the cell in this manner is unduly complicated and requires an impractical amount of equipment. For example, the evacuation of the cell, the melting of the two reactants, the feeding of the reactants through fill spouts and the subsequent pinching off and sealing of the reactants are time consuming and uneconomical steps, particularly if the cells in question are to be mass produced. Also, the fill spouts, which are required when the reactants are to be added in a molten state, present some additional difficulties in the preparation of arrays of such cells. Because the fill spouts, even after being pinched off and sealed protrude from the cell body itself, they interfere with the stacking of the cells in the preparation of batteries. They also provide an inherent weakness in the cells inasmuch as damage to the fill spout will result in leakage and/or contamination of the individual cells.
It has been suggested to employ a precast composite of sulfur a porous conductive electrode rather than filling the cathodic reaction zone with molten sulfur during actual cell preparation. However, even when such a precast sulfur electrode is employed, the prior art method for the preparation of sodium/sulfur cells suffers from many of the aforementioned deficiencies. This is the case since prior art techniques still call for the use of a vacuum to evacuate the cell subassembly and the addition of sodium to the cell in a molten form through a fill tube which is subsequently pinched off and sealed.
It is an object of this invention to provide a method for the preparation of a sodium/sulfur cell wherein the reactants, i.e., sodium and sulfur, need not be added to the cell subassembly in a molten state and wherein the cell does not require the use of fill tubes which must be pinched off and sealed.